Utils: ProcessHelper: Fix running subprocess with different ENV

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Utils: ProcessHelper: Add run_subprocess_env() function

Signed-off-by: Tomas Hozza <thozza@redhat.com>
BuildHelper: rpmbuild: Fix building with changed HOME

Signed-off-by: Tomas Hozza <thozza@redhat.com>
BuildHelper: rpmbuild: TODO: check dependencies

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Utils: Document ProcessHelper functions

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Utils: use library devnull, rather than hardcoded

Signed-off-by: Tomas Hozza <thozza@redhat.com>